Thermodynamic, Spectroscopic, and Computational Studies of f-Element Complexation by N-Hydroxyethyl-diethylenetriamine-N,N′,N″,N″-tetraacetic Acid

Abstract

Potentiometric and spectroscopic techniques were combined with DFT calculations to probe the coordination environment and determine thermodynamic features of trivalent f-element complexation by N-hydroxyethyl-diethylenetriamine-N,N',N″,N″-tetraacetic acid, HEDTTA. Ligand protonation constants and lanthanide stability constants were determined using potentiometry. Five protonation constants were accessible in I = 2.0 M (H/Na)ClO. UV-vis spectroscopy was used to determine stability constants for Nd and Am complexation with HEDTTA. Luminescence spectroscopy indicates two water molecules in the inner coordination sphere of the Eu/HEDTTA complex, suggesting HEDTTA is heptadentate. Luminescence data was supported by DFT calculations, which demonstrate that substitution of the acetate pendant arm by a N-hydroxyethyl group weakens the metal-nitrogen bond. This bond elongation is reflected in HEDTTA's ability to differentiate trivalent actinides from trivalent lanthanides. The trans-lanthanide Ln/HEDTTA complex stability trend is analogous to Ln/DTPA complexation; however, the loss of one chelate ring resulting from structural substitution weakens the complexation by ∼3 orders of magnitude. Successful separation of trivalent americium from trivalent lanthanides was demonstrated when HEDTTA was utilized as aqueous holdback complexant in a liquid-liquid system. Time-dependent extraction studies for HEDTTA were compared to diethylenetriamine-N,N,N',N″,N″-pentaacetic acid (DTPA) and N-hydroxyethyl-ethylenediamine-N,N',N'-triacetic acid (HEDTA). The results indicate substantially enhanced phase-transfer kinetic rates for mixtures containing HEDTTA.

Characterization of Americium and Curium Complexes with the Protein Lanmodulin: A Potential Macromolecular Mechanism for Actinide Mobility in the Environment

“…The binding of LanM to Am 3+ was also tested under acidic conditions, since transuranic wastes derived from nuclear fuel reprocessing activities are often acidic. Upon acidification of a Am 3 LanM sample, no significant change is observed for the 5f → 5f absorbance band until about pH 3; at pH below 3, the absorbance bands shift to 503.2 nm (Figure C), consistent with spectra previously reported for uncomplexed Am 3+ in acidic media. , Fitting of the spectral variations at three different wavelengths using the Hill equation (Figure D) yields a pH 1/2 value of 2.79 ± 0.02. The absorbance spectra in the region that corresponds to the protein (<350 nm; Figure S1) also show that LanM releases the metal below pH 3, which is consistent with the changes observed for the americium-specific absorbance band.…”

“…Upon sequential additions of LanM to an Am 3+ solution at pH 5 (acetate buffer), the 5f → 5f absorbance band progressively shifts from 504.0 to 505.2 nm, which is direct evidence that LanM binds to Am 3+ in solution (Figure A). A minor feature also appears at 512–520 nm, similar to what has been observed for strong synthetic Am 3+ chelating agents containing multiple carboxylate binding moieties. , While these absorbance features have been previously − used to study small-molecule complexes of Am 3+ , they have not yet been leveraged to study an americium-protein complex. To the best of our knowledge, spectroscopic techniques specific to americium have never been used to observed its direct binding to a protein.…”

Engineering lanmodulin's selectivity for actinides over lanthanides by controlling solvent coordination and second-sphere interactions

“…It also demonstrates the crucial importance of second-sphere interactions with at least one of those waters impacting selectivity and affinity. Whereas other studies 20,21,24,26 have focused on first-coordination-sphere effects to alter An 3+ /Ln 3+ selectivity of ligands, here we show that LanM's selectivity can also be substantially enhanced by modulation of a single, second-sphere hydrogen bond.…”

Determination of Acid–Base and Complexing Parameters of Chlorine-Substituted Trifluorobenzoylacetone in Water Medium

“…As can be seen in the above table, the “true” equilibrium stability constants lie in the range from 4.5 to 11.8 logarithmic units and increased in the following order: Ce < Nd < La < Sm < Pr < Eu < Gd < Tb ≈ Dy ≈ Ho < Er < Tm < Yb ≈ Lu < Y < Sc. Table reports a comparison of data obtained and the formation constants for various REE complexes. − …”